1. Field of the Invention
The present invention relates to a method and system for polarizing the nuclei of a solid compound via spin transfer from an optically-pumped alkali vapor and use of the method in nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI).
2. Description of the Related Art
Since the discovery of nuclear magnetic resonance (NMR), attempts have been made to increase NMR signals by artificially aligning nuclear spins to a higher degree than the statistical limit set by thermal equilibrium in a magnetic field. This is motivated by the fundamentally low sensitivity of nuclear magnetic resonance. Even in the large magnetic fields used in magnetic resonance imaging (MRI), the interaction energies between atomic nuclei and the external field are dwarfed by thermal energies. In a magnetic field of 1.5 Tesla (a value commonly used for MRI) the coupling energy between a proton spin and the external field is:ΔE=ℏγBext≈2.6×10−7 eV  (1)roughly five orders of magnitude smaller than thermal energy at room temperature. Here ΔE refers to the difference in energy between a spin-up (aligned with the external field) and a spin-down (anti-aligned) proton; ℏ is the reduced Planck constant; γ refers to the proton's gyromagnetic ratio (roughly 42.6 MHz/Tesla); and Bext refers to the magnitude of the externally applied field. Because this coupling is so small, the thermally-induced polarization, given by:
                                          P            o                    ≡                                                    N                ↑                            -                              N                ↓                                                                    N                ↑                            +                              N                ↓                                                    =                              tanh            ⁡                          (                                                γ                  ⁢                                                                          ⁢                  h                  ⁢                                                                          ⁢                                      B                    0                                                                    2                  ⁢                                      k                    B                                    ⁢                  T                                            )                                -                                    (        2        )            is only 5×10−6. The polarization is by definition a measure of the difference in population between aligned nuclear spins (N↑) and anti-aligned spins N↓, normalized by their sum. Here kB refers to Boltzmann's constant. Accordingly, kBT is the thermal energy at temperature T. This result implies that only 5 nuclei per million are visible to the MRI instrument. Solutions for increasing this number, namely lowering the temperature dramatically or increasing the magnetic field, are prohibited in most circumstances such as diagnostic medical imaging. Accordingly, much research has focused on ways to polarize nuclei above this equilibrium limit.
It has been found that angular momentum can be transferred effectively from circularly-polarized light to atoms via resonant absorption, as described in A. Kastler, Science 158, 214 (1967). Since nuclei have transitional energies which are inaccessible by optical photons, one common scheme for polarization transfer is to pump electronic transitions with circularly-polarized light and then rely upon these polarized electrons to carry angular momentum to the nuclei of interest, which may or may not be within the same atom. Nuclei within the same atom can be polarized by the atom's own hyperfine coupling, which is the fundamental interaction between the electron and the nucleus. FIG. 1A is a 87Rb level diagram which illustrates D1 pumping with circularly polarized light in low magnetic field. Because of the strong hyperfine coupling between the electron and the nucleus, states are represented in the total angular momentum basis |f,mf. Light absorption promotes the atom to one of the excited 2P1/2 states (not shown). The hyperfine splitting of 6.8 GHz is shown between the f=1 and f=2 ground-state sublevels (13 and 12, respectively). Because of conservation of angular momentum, σ+ light shown by arrows 11 cannot be absorbed by the maximum angular momentum sublevel 14. Spontaneous decay from the excited state (not shown) repopulates the ground states 12 and 13 evenly, so the atoms are soon concentrated in the only state which is not depopulated which is the |2,2 state 14 as circled in FIG. 1A.
Nuclei in external atoms can be polarized through collisional interactions with the polarized alkali electron, shown in FIG. 1B. This process is called spin-exchange optical pumping (SEOP), and has been used successfully to polarize noble gas nuclei, as described in W. Happer, Annales de Physique 10, 645 (1985); J. C. Leawoods, D. A. Yablonskiy, B. Saam, D. S. Gierada, and M. S. Conradi, Concepts in Magnetic Resonance 13, 277 (2001); and S. Appelt, A. B.-A. Baranga, C. J. Erickson, M. V. Romalis, A. R. Young, and W. Happer, Physical Review A 58, 1412 (1998). Spin transfer from 87Rb electron to a 129Xe nuclei occurs through spin exchange within a Rb—Xe van der Vaals molecule. SEOP can produce noble gases with nuclear polarizations approaching 100%, increasing the NMR signals of these gases by four to five orders of magnitude over the thermal limit, as described in B. Chann, E. Babcock, L. W. Anderson, T. G. Walker, W. C. Chen, T. B. Smith, A. K. Thompson, and T. R., Gentile, Journal of Applied Physics 94, 6908 (2003) and I. C. Ruset, S. Ketel, and F. W. Hersman, Physical Review Letters 96 (2006). Such polarizations are useful in many applications, ranging from precise atomic physics to spectroscopy to medical diagnostics and imaging.
Additional mechanisms of spin transfer have been implemented in research, including direct optical pumping of semiconductors and crystals, photo-induced production of polarized radicals in biomolecules, parahydrogen-induced polarization (PHIP), and dynamic nuclear polarization (DNP) techniques.
Although spin-exchange optical pumping has been remarkably successful, its only implementation has been to polarize the nuclei of noble gases. Yet because of their very nature, noble gases are largely inert which makes polarized noble gases unsuitable for the study of chemical interactions. Further, because of their low density in gaseous phase, it is difficult to generate large masses of the polarized materials. In the case of xenon, one conventional solution to this problem is to use a continuous-flow system with a cold trap which freezes the polarized gas. Once a large enough mass of xenon has been collected in the cold trap, it can be thawed and used in experiment. However, this freezing/thawing cycle can be an additional source of polarization loss, as described in N. N. Kuzma, B. Patton, K. Raman, and W. Happer, Physical Review Letters 88, 147602 (2002).
Another technique known as dynamic nuclear polarization (DNP) can be used to polarize nuclei in a solution sample, but this process requires special paramagnetic agents to be synthesized and admixed with the sample, as described in J. H. Ardenkjaer-Larsen, B. Fridlund, A. Gram, G. Hansson, L. Hansson, M. H. Lerche, R. Servin, M. Thaning, and K. Golman, Proceedings of the National Academy of Sciences of the United States of America 100, 10158 (2003). The mixture must then be cooled to 4 kelvin in a high magnetic field and irradiated with high-power microwave radiation to transfer electron spin polarization to the nuclear spins. The sample must then be thawed to room temperature very rapidly to prevent longitudinal spin relaxation from destroying the nuclear polarization. Finally, the paramagnetic polarization agent must be removed from the sample before it can be used in MRI or NMR.
It is desirable to provide a nuclear polarization technique which polarizes large amounts of a solid compound of chemical interest while still relying upon angular momentum transfer from an optically pumped alkali vapor, since optical pumping is an inexpensive and experimentally simple process.